Apparatus and method for establishing an operating parameter for a power
supply device

Abstract

An apparatus for establishing an operating parameter for a power supply
device having an output includes: (a) a first signal source; (b) a second
signal source; (c) a third signal source; and (d) a state device. The
first signal source is controllable for generating a programming signal.
The second signal source generates a load indicating signal and is
connected with the power supply. The third signal source generates an
offset signal. The state device has a first input and a second input and
changes state when the first input has a predetermined relationship with
the second input. The first input is determined by relative values of the
programming signal and the offset signal. The second input is related with
the output. The power supply device shuts down when the state device
changes state. The method includes the steps of: (a) providing, in no
particular order, the following signals: (1) a programming signal
appropriate for the shutdown circumstance; and (2) an offset signal; (b)
applying a signal representative of the output to a first input of a state
device; (c) substantially simultaneously with step (b), applying one of
the following signals to a second input of the state device: (1) the
programming signal; or (2) a combination of the programming signal and the
offset signal; and (d) changing state of the state device when the first
input has a predetermined relationship with the second input. The shutdown
circumstance is effected when the state changes.

1. An apparatus for establishing an operating parameter for a power supply
device said power supply device having an output; the apparatus
comprising:

(a) a first signal source, said first signal source generating a first
signal,

(b) a second signal source, said second signal source generating a second
signal; said second signal source being connected with said power supply
device;

(c) a third signal source, said third signal source generating a third
signal; and

(d) a state device;

said state device having a plurality of inputs, said state device changing
state when a first input of said plurality of inputs has a predetermined
relationship with a second input of said plurality of inputs; said first
signal being adjustable selected by a user, said first input being
determined by relative values of said first signal and said third signal;
said second input being determined by said second signal, said third
signal being combined with said first signal when said first signal is
substantially equal to or exceeds said third signal.

2. An apparatus for establishing an operating parameter for a power supply
device as recited in claim 4 wherein said third signal is a constant
signal.

3. An apparatus for establishing an operating parameter for a power supply
device as recited in claim 4 wherein said third signal is derived from
said output.

4. An apparatus for establishing a shutdown circumstance for a power supply
device; said power supply device having an output; the apparatus
comprising:

(a) a first signal source, said first signal source being controllable for
selectively generating a programming signal;

(b) a second signal source; said second signal source generating a load
indicating signal; said second signal source being connected with said
power supply device;

(c) a third signal source, said third signal source generating an offset
signal; and

(d) a state device;

said state device having a first input and a second input: said state
device changing state when said first input has a predetermined
relationship with said second input; said first input being determined by
relative values of said programming signal and said offset signal; said
second input being determined by said load indicating signal: said power
supply device shutting down when said state device changes state in a
predetermined manner; said offset signal being combined with said
programming signal when said programming signal is substantially equal to
or exceeds said offset signal.

5. An apparatus for establishing a shutdown circumstance for a power supply
device as recited in claim 4 wherein said offset signal is a constant
signal.

6. An apparatus for establishing a shutdown circumstance for a power supply
device as recited in claim 4 wherein said offset signal is derived from
said output.

7. A method for establishing a shutdown circumstance for a power supply
device; the method comprising the steps of:

(a) providing, in no particular order, the following signals:

(1) a programming signal appropriate for said shutdown circumstance; and

(2) an offset signal;

(b) applying a signal representative of said output to a first input of a
state device;

(c) substantially simultaneously with step (b), applying one of the
following signals to a second input of said state device:

(1) said programming signal; or

(2) a combination of said programming signal and said offset signal, and

(d) changing state of said state device when said first input has a
predetermined relationship with said second input; said shutdown
circumstance being effected when said state changes in a predetermined
manner, said offset signal being combined with said programming signal
when said programming signal is substantially equal to or exceeds said
offset signal.

8. A method for establishing a shutdown circumstance for a power supply
device as recited in claim 7 wherein said offset signal is a constant
signal.

9. A method for establishing a shutdown circumstance for a power supply
device as recited in claim 7 wherein said offset signal is derived from
said output.

Description

BACKGROUND OF THE INVENTION

The present invention is directed to an apparatus and method for
establishing an operating parameter for electrical power supply
apparatuses. The present invention is especially directed to an apparatus
and method for establishing shutdown output current for DC-to-DC power
converter apparatuses. In most contemporary DC-to-DC power converter
apparatuses, there is an inherent current limit involved in the operation
of the apparatus. That is, beyond a certain point, the power generated by
the converter device becomes substantially constant, so as output or load
current (I) increases, the output voltage (V) decreases. When this
condition is reached, it is frequently desireable for the apparatus to
turn off. Turning off is desireable because the low output voltage is not
adequate for the load, and the increased output current can harm the
DC-to-DC converter. It is desirable for DC-to-DC power converter
apparatuses to be flexible in their applicability to various products.
Such flexibility allows a manufacturer of such apparatuses to reduce the
number of discrete apparatus models that must be offered in order to
provide a product line that addresses a wide range of possible
applications. One aspect of such desired flexibility is to provide users,
or customers, with a capability to control the output current limit for
DC-to-DC power converter apparatuses. That is, users of DC-to-DC
apparatuses desire that they may set the current limit for the apparatus.
Such control has been made available to users of such apparatuses in the
past, but there are problems with such earlier offerings, especially at
low output current levels.

Earlier solutions to providing customer, or user control of the current
limit for DC-to-DC power converter apparatuses involved an estimating
methodology that introduced significant error into the turn-off point of
the apparatus and risked uncontrolled, and therefore unanticipated shut
down of the apparatus. Such earlier solutions introduced an offset to a
programming signal in order to avoid nuisance shut down occurrences at low
current level settings. The offset thus introduced adversely affected the
accuracy of the apparatus response over a significant range of operation.

There is a need for an improved user programmable adaptive current shutdown
method and apparatus for use with power supply apparatuses. Such a method
and apparatus is especially needed in connection with DC-to-DC power
converters at low output current levels.

SUMMARY OF THE INVENTION

An apparatus for establishing an operating parameter, such as a shutdown
circumstance, for a power supply device having an output. The apparatus
comprises: (a) a first signal source; (b) a second signal source; (c) a
third signal source; and (d) a state device. The first signal source is
controllable for selectively generating a programming signal. The second
signal source generates a load indicating signal and is connected with the
power supply device. The third signal source generates an offset signal.
The state device has a first input and a second input and changes state
when the first input has a predetermined relationship with the second
input. The first input is determined by relative values of the programming
signal and the offset signal. The offset signal may be a constant value or
it may be related with the output of the power supply device. The power
supply device shuts down when the state device changes state in a
predetermined manner.

The method of the present invention comprises the steps of: (a) providing,
in no particular order, the following signals: (1) a programming signal
appropriate for the shutdown circumstance; and (2) an offset signal; (b)
applying a signal representative of the output to a first input of a state
device; (c) substantially simultaneously with step (b), applying one of
the following signals to a second input of the state device: (1) the
programming signal; or (2) a combination of the programming signal and the
offset signal; and (d) changing state of the state device when the first
input has a predetermined relationship with the second input. The shutdown
circumstance is effected when the state changes in a predetermined manner.

The invention is particularly suited for user-programming of output current
limits for DC-to-DC power converter devices. Present such programming
capabilities employing prior art apparatuses and methods introduce
programming errors because a fixed offset voltage is imposed upon
programming voltages in order to avoid a situation where the power
converter device is "locked out" and cannot turn on.

It would be useful to have an apparatus and method for programming DC-to-DC
power converter shutdown current parameter levels in a manner that
diminishes programming errors and still avoids placing a power converter
device in a "lock out" state where it is unable to turn on.

It is, therefore, an object of the present invention to provide an
apparatus and method for programming a DC-to-DC power converter's shutdown
current with diminished programming errors as compared with prior art
apparatuses and methods.

It is a fuirther object of the present invention to provide an apparatus
and method for programming a DC-to-DC power converter's shutdown current
without placing the power converter in a "lock out" state, unable to turn
on.

Further objects and features of the present invention will be apparent from
the following specification and claims when considered in connection with
the accompanying drawings, in which like elements are labeled using like
reference numerals in the various figures, illustrating the preferred
embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical schematic diagram of a prior art apparatus for
current level shutdown programming.

FIG. 2 is an electrical schematic diagram of a first embodiment of an
apparatus for current level shutdown programming according to the present
invention.

FIG. 3 is an electrical schematic diagram of a second embodiment of an
apparatus for current level shutdown programming according to the present
invention.

FIG. 4 is a graphic representation of the relationship between programming
current and shutdown current for prior art apparatuses and for the
apparatus of the present invention.

FIG. 5 is a flow chart illustrating the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an electrical schematic diagram of a prior art apparatus for
current level shutdown programming. In FIG. 1, a shutdown programming
apparatus 10 includes a state device 12 with a first input 14 and a second
input 16. An output 18 of state device 12 changes state, as indicated by
the waveform "SHUTDOWN" in FIG. 1, whenever signals appearing at first
input 14 have a predetermined relationship with signals appearing at
second input 16. For example, when state device 12 is embodied in a
comparator-type device, output 18 will change state from a low state to a
high state when value of a signal appearing at first input 14 is less than
value of a signal appearing at second input 16. Output 18 is connected
with a host device, not shown in FIG. 1, such as a power converter in a
manner that configures the host device to alter its operation when state
device 12 changes state in a particular manner, for example from a low
state to a high state. For example, the host device may be connected with
output 18 to cause the host device to shut down when state device 12
changes state in a particular manner. It is such an arrangement that is
contemplated as the preferred embodiment of the present invention: an
apparatus (e.g., apparatus 10) connected with a power supply device in a
manner to cause the power supply device to shut down when state device 12
changes state in a particular manner.

In prior art apparatus 10, a signal representative of the load of the host
device is applied to second input 16, such as voltage V.sub.load, which is
proportional to the load current I.sub.load. First input 14 receives a
signal from a programming circuit 20. Programming circuit 20 includes a
programming signal source 22, a summing node 24, an error signal source
26, an amplifying unit 28, and a load 30.

Programming signal source 22 may be configured as a ladder-type circuit
from which an operator may select a programming signal, such as
programming signal V.sub.prog, from among a plurality of discrete choices
of programming signal level. The choice of which level of programming
signal V.sub.prog to employ may also be effected using other circuit or
software arrangements. The chosen level of programming signal V.sub.prog
is determinative of the parameter value of a selected parameter associated
with the host device (not shown in FIG. 1) when the host device shuts
down. For example, choosing a particular value of programming signal
V.sub.prog may determine the value of current provided at the load of the
host device at the point at which the host device shuts down; the shutdown
current of the power supply. Programming signal V.sub.prog is applied to
an additive input 32 of summing node 24. Apparatus 10 and its associated
host device (not shown in FIG. 1) are preferably arranged so that:

V.sub.load.varies.I.sub.load [1]

V.sub.prog.varies.I.sub.prog [2]

That is, load voltage V.sub.load is proportional to I.sub.load (current
through the load of the host device), and programming signal V.sub.prog is
proportional to I.sub.prog (current through programming circuit 20).
Moreover, it is preferable that programming current signal I.sub.prog be
related to load current I.sub.load in order that programming signal
V.sub.prog (and, hence according to expression [2], programming current
signal I.sub.prog) be useful in reliably establishing shutdown current in
the host device.

An error signal, such as error signal V.sub.err is applied from error
signal source 26 to a subtractive input 34 of summing node 24. Error
signal V.sub.err is intended as an offset value to ensure that the
response of the host device does not approach a "lock-out" condition where
the host device cannot turn on. Such a "lock-out" condition would exist,
for example when programming signal V.sub.prog is set so low that state
device 12 will never be in a state allowing the host device to turn on at
any acceptable level of load current (I.sub.load). Stated another way, as
a practical matter, there is a design lower limit for load current
designed into the host device, and a lock-out condition exists whenever
programming signal V.sub.prog sets shut down current levels below that
design lower limit for load current.

An output 36 carries a signal which is substantially equal to (V.sub.prog
+V.sub.err), and that signal is applied to an input 38 of amplifying unit
28. If, by way of example, amplifying unit 28 has a gain of k3, then a
signal produced at an output 40 of amplifying unit 28 will have
substantially the value k.sub.3 (V.sub.prog +V.sub.err). That signal is
represented as a signal V.sub.Comp in FIG. 1. Therefore, to summarize, in
FIG. 1:

V.sub.comp =k.sub.3 (V.sub.prog +V.sub.err) [3]

Since V.sub.err is a constant value signal, expression [3] may be rewritten
to reflect the constant value of (k.sub.3.multidot.V.sub.err):

V.sub.comp =k.sub.3.multidot.I.sub.prog +k.sub.4 [4]

where k.sub.4 is a constant

The introduction of constant value error signal V.sub.err introduces an
unacceptable degree of error in correlating programming current I.sub.prog
with shutdown current for the host device. It is this correlating error
that is obviated by the present invention.

FIG. 2 is an electrical schematic diagram of a first embodiment of an
apparatus for current level shutdown programming according to the present
invention. In FIG. 2, a shutdown programming apparatus 50 includes a state
device 52 with a first input 54 and a second input 56. An output 58 of
state device 52 changes state, as indicated by the waveform "SHUTDOWN" in
FIG. 2, whenever signals appearing at first input 54 have a predetermined
relationship with signals appearing at second input 56. For example, when
state device 52 is embodied in a comparator-type device, output 58 will
change state from a low state to a high state when value of a signal
appearing at first input 54 is less than value of a signal appearing at
second input 56. Output 58 is connected with a host device, not shown in
FIG. 2, such as a power converter in a manner that configures the host
device to alter its operation when state device 52 changes state in a
manner substantially the same as a host device responds to state changes
effected by apparatus 10 (FIG. 1). In order to avoid prolixity, the
relationship between host device and the apparatus of the present
invention for programming shutdown current in the host device will not be
repeated here.

In apparatus 50, a signal representative of the load of the host device is
applied to second input 56, such as load voltage V.sub.load. First input
54 receives a signal from a programming circuit 60. Programming circuit 60
includes a programming signal source 62, an amplifying unit 64, a load 66,
a reference signal source 68, and a circuit control device 70.

Programming signal source 62 may be configured in a manner similar to
programming signal source 22 (FIG. 1). The chosen level of programming
signal V.sub.prog is determinative of a selected parameter associated with
the host device (not shown in FIG. 2) when the host device shuts down,
such as shutdown current at the load of the host device. Programming
signal V.sub.prog is applied to amplifying unit 64. Apparatus 50 and its
associated host device (not shown in FIG. 2) are preferably arranged so
that expressions [1] and [2] are valid:

V.sub.load.varies.I.sub.load [1]

V.sub.prog.varies.I.sub.prog [2]

A reference signal, such as reference signal V.sub.ref is applied from
reference signal source 68 to circuit control device 70. Reference signal
V.sub.ref is intended as an offset value to ensure that the response of
the host device does not approach a lock-out condition. Circuit control
device 70 may preferably be embodied in a diode, as indicated in FIG. 2.

If, by way of example, amplifying unit 64 has a gain of k.sub.3, then a
signal produced at an output 65 of amplifying unit 64 will have
substantially the value (k.sub.3.multidot.V.sub.prog), and is applied to
first input 54 via a resistor 66 having a value of R.sub.3. Circuit
control device 70 operates to apply reference voltage V.sub.ref to first
input 54 via a resistor 67 when signal (k.sub.5.multidot.V.sub.ref) is
greater than signal (k.sub.3.multidot.V.sub.prog). Resistor 67 has a value
of R.sub.5. Constant value k.sub.5 is defined below in expression [6]. As
a result, a voltage V.sub.comp1 is applied to first input 54 which is a
combination of derivatives of reference signal V.sub.ref and programming
signal V.sub.prog in the following proportions:
##EQU1##

If resistor 67 is shorted, then value R.sub.5 =0 and the result is that
voltage V.sub.comp1 =V.sub.ref.

Otherwise, when signal (k.sub.5.multidot.V.sub.ref) is less than signal
(k.sub.3.multidot.V.sub.prog), voltage V.sub.comp1 applied to first input
54 of state device 52 equals signal (k.sub.3.multidot.V.sub.prog). For
purposes of illustration, all of these various signal relationships assume
control device 70 operates as an ideal diode.

Thus, reference voltage V.sub.ref is not always involved in signal
V.sub.comp1 applied by programming circuit 60 to first input 54 of state
device 52. The offset provided by reference voltage V.sub.ref is only
involved in operation of apparatus 50 when the programming signal
V.sub.prog is sufficiently small to cause the value
(k.sub.3.multidot.V.sub.prog to be less than the value
(k.sub.5.multidot.V.sub.ref). This selective involvement of an offset
provided by reference voltage V.sub.ref significantly reduces introduction
of programming error throughout the operating range of the host device
associated with apparatus 50; the selective application of reference
voltage V.sub.ref to operating ranges of apparatus 50 having low levels of
programming signal V.sub.prog provides protection from placing apparatus
50 in a "lock-out" condition while avoiding introduction of unnecessary
programming errors in the remainder of the operating range of the host
device associated with apparatus 50.

Therefore, to summarize, in FIG. 2:
##EQU2##

V.sub.comp1 =(k.sub.3.multidot.V.sub.prog) [8]

when (k.sub.3.multidot.V.sub.prog)>k.sub.5.multidot.V.sub.ref

That is, offset provided by imposing reference voltage V.sub.ref into
signal V.sub.comp1 only at low values of programming signal V.sub.prog
provides a close correlation (i.e., with reduced error) between
programming current I.sub.prog and shutdown current in values of
programming signal V.sub.prog greater than reference voltage V.sub.ref.

FIG. 3 is an electrical schematic diagram of a second embodiment of an
apparatus for current level shutdown programming according to the present
invention. In FIG. 3, a shutdown programming apparatus 80 includes a state
device 82 with a first input 84 and a second input 86. An output 88 of
state device 82 changes state, as indicated by the waveform "SHUTDOWN" in
FIG. 3, whenever signals appearing at first input 84 have a predetermined
relationship with signals appearing at second input 86. In apparatus 80, a
signal representative of the load of the host device is applied to second
input 86, such as load voltage V.sub.load. First input 84 receives a
signal from a programming circuit 90. Programming circuit 90 includes a
programming signal source 92, an amplifying unit 94, a load 96, an
adjustment signal source 98, and a circuit control device 100.

Comparison of the embodiments of the present invention illustrated in FIGS.
2 and 3 reveals that the differences between the embodiments substantially
arise in the configurations of programming circuit 60 (FIG. 2) and
programming circuit 90 (FIG. 3). In order to avoid prolixity, portions of
apparatus 80 which are substantially similar in configuration and
operation to apparatus 50 (FIG. 2) will not be repeated here.

Apparatus 80 and an associated host device (not shown in FIG. 3) are
preferably arranged so that expressions [1] and [2] are valid:

V.sub.load.varies.I.sub.load [1]

V.sub.prog.varies.I.sub.prog [2]

An adjustment signal such as adjustment signal V.sub.adj is applied from
adjustment signal source 98 to circuit control device 100. Adjustment
signal source 98 includes an amplifier device 110 with a feedback resistor
112 and an input bias resistor 114. Input bias resistor 114 is connected
to convey load voltage V.sub.load to a noninverting input 116 of amplifier
device 110. A voltage V.sub.1 is applied to an inverting input 118 of
amplifier device 110. An output 120 of amplifier device 110 conveys
adjustment signal V.sub.adj to circuit control device 100. Adjustment
signal V.sub.adj is intended as an offset value to ensure that the
response of the host device does not approach a lock-out condition.
Circuit control device 100 may be preferably embodied in a diode, as
indicated in FIG. 3.

If, by way of example, amplifying unit 94 has a gain of k.sub.3, then a
signal produced at an output 95 of amplifying unit 94 will have
substantially the value (k.sub.3.multidot.V.sub.prog), and is applied to
first input 84 via a resistor 96 having a value of R.sub.3. Circuit
control device 100 operates to apply an adjustment signal V.sub.adj to
first input 84 via a resistor 97 when signal (k.sub.5.multidot.V.sub.adj)
is greater than signal (k.sub.3.multidot.V.sub.prog). Resistor 97 has a
value of R.sub.5. Constant value k.sub.5 is defined below in expression
[9]. As a result, a voltage V.sub.comp2 is applied to first input 84 which
is a combination of derivatives of reference signal V.sub.adj and
programming signal V.sub.prog in the following proportions:
##EQU3##

If resistor 97 is shorted, then value R.sub.5 =0 and the result is that
voltage V.sub.comp2 =V.sub.adj.

Otherwise, when signal (k.sub.5.multidot.V.sub.adj) is less than signal
(k.sub.3.multidot.V.sub.prog), voltage V.sub.comp2 applied to first input
84 of state device 82 equals signal (k.sub.3.multidot.V.sub.prog). For
purposes of illustration, all of these various signal relatoinships assume
control device 100 operates as an ideal diode.

Thus, adjustment signal V.sub.adj is not always involved in the signal
applied by programming circuit 80 to first input 84 of state device 82.
The offset provided by adjustment signal V.sub.adj is only involved in
operation of apparatus 80 when the programming voltage signal V.sub.prog
is sufficiently small to cause the value (k.sub.3.multidot.V.sub.prog) to
be less than the value (k.sub.5.multidot.V.sub.adj). This selective
involvement of offset signal V.sub.adj avoids introduction of programming
error throughout the operating range of apparatus 80 in a manner similar
to the operation of apparatus 50 (FIG. 2). By deriving adjustment signal
V.sub.adj from load voltage V.sub.load the offset provided by adjustment
signal V.sub.adj for operation of apparatus 80 is more dynamically
responsive to the host device associated with apparatus 80 than was the
case involving apparatus 50 (FIG. 2). It is because of the added dynamic
response of the embodiment of the present invention illustrated in FIG. 3
that the embodiment of FIG. 3 is regarded as the preferred embodiment of
the present invention.

To summarize, in FIG. 3:
##EQU4##

V.sub.comp2 =(k.sub.3.multidot.V.sub.prog) [12]

when (k.sub.3.multidot.V.sub.prod >k.sub.5.multidot.V.sub.adj

When resistor 112 has a value of R.sub.1, and resistor 114 has a value of
R.sub.2, then it may be concluded that:
##EQU5##

Noting that V.sub.1, R.sub.1 and R.sub.2 are each constant values,
expression [13] may be reduced to:
##EQU6##

FIG. 4 is a graphic representation of the relationship between programming
current and shutdown current for prior art apparatuses and for the
apparatus of the present invention. In FIG. 4, a graphic plot 130 displays
shutdown current (I.sub.shut) for a host device appropriate for use with
the present invention plotted vis-a-vis a vertical axis 132. Shutdown
current I.sub.shut is a function of programming current (I.sub.prog),
plotted vis-a-vis a horizontal axis 134. A dotted-line plot 136 extends
generally linearly from a minimum intercept 138 on axis 132. The distance
from minimum intercept 132 to the origin 140 of plot 130 is the offset
provided by prior art and present invention apparatuses to avoid putting
host devices in a "lock-out" condition. That is, design minimum shutdown
current I.sub.shut for the host device used with the apparatus of the
present invention is set at a value between origin 140 and minimum
intercept 138 on axis 132.

If a host device is allowed to approach or reach origin 140, the programmed
shutdown current I.sub.shut will be below the design minimum shutdown
current; in such a condition the host device will not be able to turn on.
This "lock-out" condition is known to those skilled in the art. As a
generally accepted engineering good practice, a margin is provided to
ensure that design minimum shutdown current is not approached, thereby
obviating any risk of a "lock-out" condition in a host device.

An unfortunate consequence of the constant offset provided by the prior art
apparatus (FIG. 1) is that the departure point of plot 136 (V.sub.comp,
FIG. 1) is offset from origin 140 and the slope of plot 136 is thereby
affected. The change in slope introduces programming errors
(representatively indicated in FIG. 4 at 142).

The present invention, in both disclosed embodiments illustrated herein
(FIGS. 1 and 2) provide a departure point for a plot from origin 140, yet
avoid approaching origin 140. This is accomplished because the offset
between origin 140 and minimum intercept 138 on axis 132 is only
introduced at low programming currents I.sub.prog. Thus, programming
errors are avoided except where desired: to ensure there is not too close
an approach to a "lock-out" condition near origin 140. The constant offset
value introduced at low programming current I.sub.prog, illustrated in
FIG. 2, is indicated as an intersection of two linear plots, and
identified as V.sub.comp1 in FIG. 4. That is, the value V.sub.ref is
additively imposed upon programming signal V.sub.prog at low values of
programming signal V.sub.prog to establish a minimum value of shutdown
current I.sub.shut at minimum intercept 138 for low values of programming
current I.sub.prog. When programming signal V.sub.prog equals or exceeds
reference voltage V.sub.ref, then the response of shut down current
conforms to a plot that originates at origin 140. In such manner,
programming errors are substantially eliminated.

FIG. 5 is a flow chart illustrating the method of the present invention. In
FIG. 5, the method begins with providing two signals in no particular
order, as indicated by a block 160. The two signals provided according to
block 160 are a programming signal, as indicated by a block 162, and an
offset signal, as indicated by a block 164. A signal representative of the
output of a host device associated with the practice of the method of the
present invention is provided according to a block 166. According to a
block 168, one of the programming signal sand a combination of the
programming signal and the offset signal (combined as indicated by a block
169) is provided. The provision of signals according to blocks 166 and 168
preferably occurs substantially simultaneously.

Signals provided according to blocks 166, 168 are applied to a state
device, as indicated by a block 170. A query is posed: "Is there a
predetermined relation between the signals applied to the state device
according to lock 170?", according to a block 172. If the predetermined
relation does not exist between the signals applied to the state device,
the process proceeds according to "NO" response path 174 and later-in-time
samples of the selected signals and applied to the state device, according
to block 170. If the predetermined relation does exist, the process
proceeds according to "YES" response path 176, and the state device
changes state, as indicated by a block 178. When the state change occurs
according to a predetermined manner, the host device, such as a power
supply device, shuts down. This last step of shutting down is not
reflected in FIG. 5.

It is to be understood that, while the detailed drawings and specific
examples given describe preferred embodiments of the invention, they are
for the purpose of illustration only, that the apparatus and method of the
invention are not limited to the precise details and conditions disclosed
and that various changes may be made therein without departing from the
spirit of the invention which is defined by the following claims: